Method of Manufacturing Cold-rolled Magnesium Alloy Sheet for Improving Formability and Cold-rolled Magnesium Alloy Sheet Having Improved Formability Manufactured thereby

ABSTRACT

A method of manufacturing a cold-rolled magnesium alloy sheet for improving formability, including, (a) preparing a composite sheet which is composed of a main sheet made of magnesium alloy and a restrainer made of steel and functioning to suppress transverse plastic deformation (plastic deformation in a transverse direction) of the main sheet during a rolling process, (b) performing a cold-rolling of the composite sheet, and (c) separating the cold-rolled magnesium alloy sheet from the cold-rolled composite sheet obtained from (b). The magnesium alloy sheets manufactured by the invention has improved formability at an ambient temperature, and thus can be usefully applied to extensive industrial fields such as automobiles and aircrafts.

CROSS REFERENCE RELATED APPLICATION

This application claims foreign priority of Korean Patent ApplicationNo. 10-2013-0026578, filed on Mar. 13, 2013, which is incorporated byreference in its entirety into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a method of manufacturing acold-rolled magnesium alloy sheet for improving formability and acold-rolled magnesium alloy sheet having improved formabilitymanufactured thereby, and more particularly, to a method ofmanufacturing a cold-rolled magnesium alloy sheet using a compositesheet including a restrainer functioning to suppress transverse plasticdeformation of a magnesium alloy sheet so as to improve formability anda cold-rolled magnesium alloy sheet having improved formabilitymanufactured thereby.

2. Description of the Related Art

A variety of sheet-working processes such as a press working are beingused for the production of sheets in industrial fields such asautomobiles and aircrafts. During a sheet-working process, a sheetworkpiece partially or completely undergoes a complicated deformationsuch as stretching, bending, flanging, deep drawing or combinationsthereof.

Therefore, in a process of working sheets, formability, which indicatesa degree to which a sheet can be plastically deformed without fracture,is critical above all physical properties.

Although a magnesium alloy is getting a lot of attention as a structuralalloy material which exhibits a low specific gravity and superiorspecific strength and stiffness, the magnesium alloy does not have asufficient number of slip systems but has a limited number of activeslip systems because it has a hexagonal close packing crystal structure.In particular, since the slip system in the basal texture, which isformed in the magnesium alloy by hot-rolling or extrusion, has a Schmidfactor close to zero for a deformation along a c-axis, formability at anambient temperature is deteriorated which inhibits its extensiveapplications.

Accordingly, in order to employ the magnesium alloy sheet in morevarious industrial fields than current application fields, there is acompelling need for development of a magnesium alloy sheet havingsuperior formability at an ambient temperature.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made keeping in mind theabove problems occurring in the prior art, and an object of the presentinvention is to provide a method of manufacturing a cold-rolledmagnesium alloy sheet for improving formability and a cold-rolledmagnesium alloy sheet having improved formability manufactured thereby.

In order to accomplish the above object, the present invention providesa method of manufacturing a cold-rolled magnesium alloy sheet forimproving formability, comprising: (a) preparing a composite sheet whichis composed of a main sheet made of magnesium alloy and a restrainermade of steel and functioning to suppress transverse plastic deformation(plastic deformation in a transverse direction) of the main sheet duringa rolling process, (b) performing a cold-rolling of the composite sheet,and (c) separating the cold-rolled magnesium alloy sheet from thecold-rolled composite sheet obtained from (b).

Furthermore, the present invention provides a cold-rolled magnesiumalloy sheet having improved formability manufactured by the method.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic view illustrating an embodiment of a compositesheet used in a method of manufacturing a cold-rolled magnesium alloysheet for improving formability, according to the present invention;

FIGS. 2A to 2C are optical photomicrographs which show microscopictextures at a center point, a ¼ point and an edge point positioned in atransverse direction (TD) across an end face of the specimen prepared inan example, in which the end face is perpendicular to the rollingdirection (RD);

FIGS. 3A and 3B are an inverse pole figure (IPF) map in a normaldirection and a (0001) pole figure which are taken at an edge area of anend face of the specimen prepared in the example using an electron backscatter diffraction (EBSD), in which the end face is perpendicular tothe rolling direction (RD);

FIG. 4 is a graph showing tension test results of magnesium alloyspecimens prepared in an example according to the present invention andin a comparative example; and

FIG. 5 is a graph showing a limit dome height (LDH) results of themagnesium alloy specimens prepared in the example according to thepresent invention and in the comparative example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be more particularly described with reference to theaccompanying drawings.

A method of manufacturing a cold rolled magnesium alloy sheet forimproving formability comprises (a) preparing a composite sheet which iscomposed of a main sheet made of magnesium alloy and a restrainer madeof steel and functioning to suppress transverse plastic deformation(plastic deformation in a transverse direction) of the main sheet duringa rolling process, (b) performing a cold-rolling of the composite sheet,and (c) separating the cold-rolled magnesium alloy sheet from thecold-rolled composite sheet obtained from the operation (b).

The operations of the method according to the present invention will nowbe more specifically described.

Operation (a) of a method according to the present invention isperformed in order to produce a composite sheet which is composed of amain sheet made of magnesium alloy and a restrainer made of steel andfunctioning to suppress transverse plastic deformation (deformation in atransverse direction) of the main sheet during a rolling process.

In this description, the composite sheet is defined as sheet-shapedproduct composed of different kinds of materials, that is, magnesium andsteel, in which the different materials are separately prepared and thencoupled to each other thus providing the sheet-shaped product.

In the composite sheet, the magnesium sheet, which is the target to beformed through a cold-rolling process, is coupled to a restrainer thatfunctions to suppress plastic deformation in a transverse direction (MD)perpendicular to a rolling direction (RD) during cold-rolling process,with the result that the magnesium sheet is contained in the compositesheet according to the present invention.

In this context, the restrainer may have any appropriate shape withoutany limitation as long as it can apply a compressive stress which iscapable of suppressing plastic deformation in a transverse direction ofthe magnesium sheet during a rolling process.

In a case where a composite sheet 1 which is configured to have a shapeand a structure, for example, as illustrated in FIG. 1, operation (a)may be fulfilled in such a way that a restrainer 3 having a groove isprepared by processing a hot-rolled sheet, a main sheet 2 is prepared byprocessing a hot-rolled magnesium alloy sheet to have dimensionscorresponding to the shape of the groove, and the magnesium sheet 2 isdisposed in the groove of the restrainer 3. In case of necessity, themain sheet 2 may be temporarily bonded to the restrainer 3 such that thefinal composite sheet can be effectively produced in a cold-rollingoperation to be described later.

The classes of magnesium alloy constituting the main sheet and steelconstituting the restrainer are not limited to any specific class aslong as they can be rolled at a desired rolling reduction and stiffnessof the steel is higher that of the magnesium alloy. When the stiffnessof the steel constituting the restrainer is higher than that of themagnesium alloy constituting the main sheet, the restrainer will apply acompressive stress against expansion of the magnesium sheet in atransverse direction during a subsequent rolling operation of thecomposite sheet, and thus rolling properties of the composite sheet canbe effectively improved when the cold-rolling operation is performed inoperation (b) to be fully described later.

Operation (b) of the method according to the present invention, which isintended to cold-roll the composite sheet which is prepared in operation(a), may be fulfilled using a well-known rolling machine and awell-known rolling technology.

In this operation, the composite sheet, which has been prepared in amanner described in detail above, is cold-rolled. Thanks to thisoperation (b), edge cracks do not occur even in the magnesium alloysheet which must be conventionally hot-rolled at a sufficiently hightemperature of higher than 230° C. in order to avoid occurrence ofcracks, and a sheet, which has an intact and fine texture even whenbeing cold-rolled at a low temperature of lower than 100° C., but higherthan ambient temperature, can be produced.

More specifically, the restrainer, which is composed of steel, applies acompressive stress to the magnesium alloy sheet in a transversedirection (TD) during a cold-rolling process to thus inhibit occurrenceof edge cracks and cause tensile twinning, with the result that aductility of the sheet can be significantly improved and thus ahigh-quality sheet can be produced even through a cold-rolling process.

According to the present invention, magnesium alloy, which is inevitablysubjected to a hot-rolling process because it has a low ductility andthus a low workability at an ambient temperature, can also bemanufactured into a high-quality sheet. Therefore, it is possible toprevent the shortening of service life of a roll caused by the heatingof working material and rolling rolls, and it is possible to reduceproduction costs owing to energy saving and thus to manufacture aneconomical and high-quality sheet.

Operation (c) of the method according to the present invention, which isintended to separate the cold-rolled magnesium alloy sheet from thecomposite sheet obtained in operation (b), may employ any solution aslong as it can separate the cold-rolled magnesium alloy sheet from therestrainer.

Since the cold-rolled magnesium alloy sheet manufactured by the methodaccording to the present invention has an ambient temperatureformability which is remarkably improved compared to the magnesium alloysheet which is not subjected to the cold-rolling process, it is expectedthat it gives advantages in economic efficiency and simplifiesprocessing in various industrial fields such as automobiles andaircrafts and thus the application field of the magnesium alloy sheet isenormously broadened.

Specifically, the cold-rolled magnesium alloy sheet having an improvedformability, according to the present invention, is characterized inthat it has a limit dome height (LDH) at an ambient temperature which isincreased by 20% or more, compared to the magnesium alloy sheet which isnot subjected to the cold-rolling process.

In this regard, LDH, which refers to an index which is extensively usedto evaluate a press-formability among formability by various processes,is defined in such a way that a disc-shaped specimen having apredetermined dimension is held at a circumference thereof by a certainforce, the specimen is deformed by a spherical punch which has apredetermined diameter and moves a predetermined speed, a deformedheight of the specimen is taken as the LDH when the specimen is broken.

The present invention will now be described in detail with reference toan example. The example is set forth to illustrate, but is not to beconstrued as the limit of the present invention.

EXAMPLE

A commercial hot-rolled AZ31 (Al: 3 weight %, Zn: 1 weight %, Mn: 0.3weight % and Mg: balance) sheet having a thickness of 1.2 mm wasprocessed to prepare a specimen having a dimension of 90 mm×12 mm×1.2mm, and a groove was formed between the opposite ends on a side of ahot-rolled steel sheet (C: 0.12 weight %, Si: 0.20 weight %, Mn: 0.87weight % and Fe: balance). The specimen was fitted in the groove of thesteel sheet to prepare a composite sheet having a shape and a structureas shown in FIG. 1.

Thereafter, the composite sheet was homogenized at a low temperature of100° C. for 20 minutes, and was then cold-rolled at a rolling reductionof 3% using a rolling machine equipped with rolls having a diameter of250 mm under the conditions of a rolling temperature of 100° C. equal tothe homogenization temperature and a rolling speed of 2 rpm.Subsequently, the magnesium alloy specimen was separated from thecold-rolled composite sheet.

FIGS. 2A to 2C are optical photomicrographs which show microscopictextures at three points (a center point, a ¼ point and an edge point)positioned in a transverse direction (TD) across an end face of thespecimen prepared in the example, in which the end face is perpendicularto the rolling direction (RD). From the optical photomicrographs, it isascertained that densities of twinning are similar to each otherregardless of measurement location.

FIGS. 3A and 3B are an inverse pole figure (IPF) map in a normaldirection and a (0001) pole figure which are taken at an edge area of anend face of the specimen prepared in the example using an electron backscatter diffraction (EBSD), in which the end face is perpendicular tothe rolling direction (RD). From the figures, it is found that most oftwinnings are of {10-12} tensile twinning. Furthermore, it is alsoascertained that a compressive stress was applied to the magnesium alloysheet in a transverse direction (TD) during the rolling process due tothe steel sheet which has a strength higher than the magnesium alloysheet in a transverse direction (TD), from the fact that face {0001} wasturned in a transverse direction (TD).

COMPARATIVE EXAMPLE

A commercial hot-rolled AZ31 (Al: 3 weight %, Zn: 1 weight %, Mn: 0.3weight % and Mg: balance) sheet having a thickness of 1.2 mm wasprocessed to prepare a specimen having a dimension of 90 mm×12 mm×1.2mm.

EXPERIMENTAL EXAMPLE

Measurement of tensile strength and formability are measured for themagnesium alloy sheet specimen prepared in the example and thecomparative example is executed as follows.

For the tension test, a tension test specimen which has a gauge lengthof 25 mm, a gauge width of 6 mm and a thickness of 1.16 mm was preparedto be configured to be extended in a rolling direction. The tension testwas executed at a strain rate of 0.001 s⁻¹ and a temperature of 100° C.,and then the measurement results were represented in FIG. 4.

For an additional LDH test, a disc-shaped specimen which has a diameterof 50 mm and a thickness 1.16 mm was prepared. The specimen wasinterposed between upper and lower dies of an LDH test equipment, andwas then held by a force of 15 kN. Subsequently, the specimen wasdeformed by a spherical punch having a diameter of 25 mm which movestoward the specimen at a speed of 0.02 mm/sec. When the specimen wasbroken, a deformed height of the specimen was measured, and themeasurement results were represented in FIG. 5.

From FIGS. 4 and 5, it is ascertained that the magnesium alloy specimenprepared in the example and the magnesium alloy specimen prepared in thecomparative example have tension strengths of 275 MPa and 272 MPa,respectively, which are almost same as each other, whereas yieldstrengths of both the specimens were 187 MPa and 144 MPa in which theyield strength of the specimen prepared in the example was increasedcompared to that of the specimen prepared in the comparative example. Asfor formability, it is ascertained that the cold-rolled magnesium alloyspecimen prepared in the example exhibits a remarkable formability whichis improved by 23% compared to the magnesium alloy specimen prepared inthe comparative example.

Although the preferred example of the present invention has beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

What is claimed is:
 1. A method of manufacturing a cold-rolled magnesiumalloy sheet for improving formability, comprising: (a) preparing acomposite sheet which is composed of a main sheet made of magnesiumalloy and a restrainer made of steel and functioning to suppresstransverse plastic deformation (plastic deformation in a transversedirection) of the main sheet during a rolling process, (b) performing acold-rolling of the composite sheet, and (c) separating the cold-rolledmagnesium alloy sheet from the cold-rolled composite sheet obtained from(b).
 2. The method according to claim 1, wherein the restrainer includesa groove formed in a side thereof across opposite ends thereof, and themain magnesium alloy sheet is disposed in the groove of the restrainer.3. The method according to claim 2, wherein both lateral sides of themain magnesium alloy sheet are in contact with mating sides of thegroove of the restrainer without gaps therebetween.
 4. The methodaccording to claim 1, wherein the main magnesium alloy sheet in (a) is ahot-rolled AZ31 sheet having a composition of Al: 3 weight %, Zn: 1weight %, Mn: 0.3 weight % and Mg: balance.
 5. The method according toclaim 1, wherein (b) is performed at a temperature of an ambienttemperature −100° C.
 6. The method according to claim 1, wherein themain magnesium alloy sheet of the composite sheet is coupled with therestrainer without a gap therebetween.
 7. A cold-rolled magnesium alloysheet having an improved formability, manufactured by a method accordingto any one of claims 1 to
 6. 8. The cold-rolled magnesium alloy sheetaccording to claim 7, wherein the cold-rolled magnesium alloy sheet hasa limit dome height (LDH) which is increased by 20% or more, compared toa magnesium alloy sheet which is not subjected to the cold-rollingprocess.